Stats are not always the most important factor in competitive. This also applies to mixed attackers and walls. While higher stats certainly mean that they will be able to do their job much better, what matters more is if the mixed options can be justified using by reasons other than "are their Atk and SpA close?".
There are a lot of cases among mixed attackers where one offensive stat is vastly superior to the other, however it still lets them bypass their checks and counters which their higher offensive stat wouldn't. For example, Jolteon and Exeggutor in Gen 1 prefer using physical moves to beat their usual check, Chansey.
Jolteon Thunderbolt vs. Chansey: 105-124 (14.9 - 17.6%) -- possible 6HKO
Jolteon Double Kick (2 hits) vs. Chansey: 186-220 (26.4 - 31.2%) -- approx. 4HKO
Exeggutor Psychic vs. Chansey: 110-130 (15.6 - 18.4%) -- possible 6HKO
Exeggutor Double-Edge vs. Chansey: 192-226 (27.3 - 32.1%) -- guaranteed 4HKO
Jolteon and Exeggutor have 45 and 30 less Attack than their Special stat, but that's not relevant because Double Kick and Double-Edge will do better than any other special move vs Chansey.
Snorlax in GSC would run Thunder/Fire Blast to hit Pokemon like Skarmory, Forretress, Cloyster, etc. which otherwise can shrug off other attacks from it. Both moves have other utility as well, like Thunder can fish for paralysis on Gengar and other switch-ins.
Snorlax Double-Edge vs. Skarmory: 54-64 (16.2 - 19.2%) -- possible 8HKO
Snorlax Thunder vs. Skarmory: 166-196 (49.8 - 58.8%) -- 76.2% chance to 2HKO after Leftovers recovery
Here, it does not really matter that Snorlax's attack is much greater than it's SpA.
Special attackers running Focus Punch to punish their switch-in Blissey is also common in older generations.
24 Atk 30 IVs Charizard Focus Punch vs. 0 HP / 252 Def Blissey: 379-446 (58.2 - 68.5%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Choice Band Alakazam Focus Punch vs. 0 HP / 252 Def Blissey: 591-696 (90.7 - 106.9%) -- 43.8% chance to OHKO
Apart from these, there are also numerous other examples like Marshadow and Primal Groudon running Hidden Power Ice for Zygarde, Mega-Salamence, among other ones. Again, their physical attack being greater still doesn't let them get rid of Zygarde as efficiently as HP Ice does.
0 SpA Life Orb Technician Marshadow Hidden Power Ice vs. 252 HP / 4 SpD Zygarde: 322-380 (76.6 - 90.4%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Life Orb Marshadow Close Combat vs. 252 HP / 252+ Def Zygarde: 156-185 (37.1 - 44%) -- 99.8% chance to 3HKO after Leftovers recovery
252+ SpA Groudon-Primal Hidden Power Ice vs. 252 HP / 4 SpD Zygarde: 336-396 (80 - 94.2%) -- guaranteed 2HKO after Leftovers recovery
0 Atk Groudon-Primal Precipice Blades vs. 252 HP / 252+ Def Zygarde: 136-162 (32.3 - 38.5%) -- 2.6% chance to 3HKO after Leftovers recovery
From all of these examples, we can thus conclude that for a mixed attacker, having high stats on both sides is not as important as having ones decent enough to perform their well.
Mixed walls tend to be scarce as it's generally better to invest on either the physical or special side instead of both. Roughly speaking, it's the same criteria as above. If they can perform both of the roles well, they can be considered a "mixed wall". They either have well-rounded defensive stats all around, or offer some kind of utility via their moves or abilities.
An ideal example would be defensive Ho-Oh. Its special defense is 64 points higher than it's base defense, so it would make sense to maximize the SpD. Interestingly, it almost prefers to run a mixed defenses set in modern generations with most of the EVs invested in physical defense. This is because of a combination of factors other than base stats: its already stellar special defense bulk, typing and Sacred Fire acting as a great deterrent for many physical attackers. Thanks to its typing, it can check physical attackers like Zygarde, Marshadow, Arceus, Koraidon, Calyrex-Ice, Necrozma-Dusk Mane and Primal Groudon (lacking Rock-type moves). However, at the same time, it is capable of taking care of special attackers like FairyCeus, DarkCeus, Xerneas, GroundCeus, SteelCeus, etc. which makes Ho-Oh an ideal mixed tank. So, available metagame threats, typing and moves play a bigger role than raw stats alone. Pokemon like Cresselia, Lugia, Registeel, Toxapex, etc. might have a lesser gap between their defensive stats but the metagame was more favourable to Ho-Oh than them.
tl;dr stats solely are not defining mixed attackers and walls; their offensive and defensive utility does.